The invention relates to liquid-crystalline media, in particular for use in liquid-crystal displays. It furthermore relates to liquid-crystal displays containing these liquid-crystalline media, in particular liquid-crystal displays having low addressing voltages. These liquid-crystal displays are operated in TN (twisted nematic) or STN mode. The addressing takes place directly or by time multiplexing at low multiplex ratios. The TN displays are preferably operated at the first Gooch and Tarry transmission minimum. The TN and STN displays are suitable, inter alia, for outdoor applications and, owing to their suitability for common applications, are also known as xe2x80x9ccommon-use displaysxe2x80x9d.
Liquid crystals are principally used as dielectrics in display devices since the optical properties of these materials can be modified in a targeted manner by the action of an electric field.
Electro-optical devices based on liquid crystals are extremely well known to the person skilled in the art and can be based on various effects. Such devices are, for example, cells having dynamic scattering, DAP or ECB cells (DAP=deformation of lined phases; ECB=electrically controlled birefringence), guest/host cells, TN cells having a twisted nematic structure, STN (supertwisted nematic) cells, SPE (superbirefringence effect) cells, OMI (optical mode interference) cells, IPS (in-plane switching) cells or cells having a cholesteric-nematic phase conversion.
The most common display devices are TN cells. These are based on the Schadt-Helfrich effect and have a twisted nematic structure. Industrial use of such TN liquid-crystal displays requires liquid-crystalline materials which satisfy a multiplicity of conditions.
In order to achieve a long service life, the liquid-crystal materials must have, for example, good chemical and thermal stability and good stability to the action of electric fields and electromagnetic radiation.
In addition, the liquid-crystal materials must have a suitable mesophase at normal operating temperatures, i.e. in the broadest possible range above and below room temperature (20xc2x0 C.). If they are intended to be suitable for outdoor applications, for example in automobiles or in avionics, they should have, in particular, good low-temperature behavior, which means, for example, that crystallization should not occur at low temperatures of xe2x88x9220xc2x0 C. In addition, the mixtures should have clearing points TN,I above 55xc2x0 C.
Operation of TN liquid-crystal displays at the first Gooch and Tarry transmission minimum favorably affects the size of the viewing angle at which the display can be viewed (U.S. Pat. No. 4,398,803). In this case, the optical anisotropy xcex94n of the liquid-crystal mixtures is selected so that the optical path difference (=product of the cell thickness d of the TN liquid-crystal display and the optical anisotropy xcex94n of the liquid-crystal mixture) is about 0.5 xcexcm. For such applications, optical anisotropies xcex94n of from about 0.06 to 0.12 are preferred.
However, TN liquid-crystal displays can also be operated at a higher Gooch and Tarry transmission minimum. If, for example, they are operated at the second transmission minimum, the optical path difference is from about 1.0 xcexcm to 1.1 xcexcm. For such applications, liquid-crystal mixtures having optical anisotropies xcex94n of from about 0.10 to 0.21 are preferred. In general, TN liquid-crystal displays operated at the second or higher transmission minimum have greater viewing-angle dependencies than TN liquid-crystal displays operated at the first transmission minimum. On the other hand, they are usually characterized by being easier to produce and thus by better production yields.
In addition, the liquid-crystal materials should have the lowest possible rotational and flow viscosities. Low rotational viscosities xcex31 favor, in particular, short response times. By contrast, low flow viscosities xcexc20 simplify the filling of TN liquid-crystal displays.
The liquid-crystal materials should give the lowest possible threshold voltages V(10,0,20) (also known as V10 (0xc2x0, 20xc2x0 C.), since the TN liquid-crystal display can then be operated at low operating voltages Vop, which can be obtained, for example, by batteries. The three indices in the threshold voltages V(10,0,20) relate to the relative contrast (here 10%), the viewing angle (here xcex8=0 degrees at "PHgr"=0 degrees) and the temperature (here 20xc2x0 C.). The threshold voltage V(10,0,20) of a liquid-crystal mixture is influenced principally by the magnitude of the dielectric anisotropy xcex94∈, where the threshold voltage is lower the higher the dielectric anisotropy of the mixture. For example, mixtures having positive dielectric anisotropy xcex94∈ are used in common-use applications. These mixtures preferably have dielectric anisotropies xcex94∈ of from 1.5 to 39.
The liquid-crystal materials should give high contrast. In order that the TN liquid-crystal display can easily be read even at various viewing angles, the viewing-angle dependence of the contrast should in addition be as low as possible.
The liquid-crystal materials themselves, i.e. with the exception of any added dichroic dyes, must not absorb in the visible spectral region, i.e. they must be colorless, in order that the TN liquid-crystal displays have good contrast for black/white images and achieve the longest possible service life.
The liquid-crystal materials should have low electroconductivity, since otherwise a considerable part of the addressing voltage is lost due to conduction processes.
High-information TN liquid-crystal displays require liquid-crystalline substances having a steep electro-optical characteristic line which are characterized by a large ratio K33/K11 of the elastic constants K33 (bend) and K11 (splay). STN displays, in particular, are used here. By contrast, such requirements are not made of liquid-crystal materials intended for use in TN liquid-crystal displays addressed at low multiplex ratios. By contrast, the use of substances having flatter electro-optical characteristic lines favors the display of grey shades.
Since no liquid-crystalline substance which satisfies all the demands simultaneously is known hitherto, liquid-crystal mixtures of from 5 to 30 compounds are generally prepared in order to obtain materials having suitable properties. The liquid-crystal materials must also be readily miscible with one another.
A major problem in the liquid-crystal displays of the prior art is the temperature dependence of the characteristic voltages, such as the threshold voltage, and thus the operating voltage. The liquid-crystal materials should have the lowest possible temperature dependence of the threshold voltage d[V10,0,T)/V(10,0,20)]/dT (or of the dielectric anisotropy d xcex94∈ (T)/dT). This has the advantage that temperature compensation of the addressing voltage can be omitted entirely or at least partly.
A known liquid-crystal mixture from the prior art which can be employed in TN liquid-crystal displays for common-use applications is, for example, E7 (Merck Ltd, Poole, England). This mixture comprises cyanobiphenyls and -terphenyls and has the following properties:
Clearing point TN,I: 60.5xc2x0 C.
Optical anisotropy xcex94n: 0.225
Viscosity: 39 cSt
Threshold voltage V(10,0,20): 1.5 V
In spite of an acceptable clearing point TN,I and relatively good viscosity, mixtures of this type are unsuitable for many applications, for example owing to their excessively high optical anisotropies xcex94n and/or excessively high threshold voltages V(10,0,20).
In order, for example, to reduce the threshold voltage V(10,0,20), highly polar nematic compounds having a terminal cyano group are added to the liquid-crystal mixtures known hitherto. However, the effective dipole moment of these compounds is reduced to a significant extent by a varying degree of antiparallel association of these molecules, meaning that a relatively large amount of polar compounds must be added. This in turn causes various disadvantages, for example unfavorable elastic properties and high viscosities of the liquid-crystal phases.
Addition of 4-cyano-3-fluorophenyl 4-alkyl-benzoates as highly polar components to ZLI-1957/5 (commercially available mixture from Merck KGAA, Darmstadt, comprising phenylcyclohexane, cyclohexylbiphenyl and biscyclohexylbiphenyl compounds and phenyl 4-cyclohexylbenzoate) allows the threshold voltage V(10,0,20) to be reduced (H. Schadt, S. M. Kelly, J. Chem. Phys. 81(3), 1514-15 (1984)), which is explained by a lower degree of association of the added compounds. However, these liquid-crystal phases likewise do not satisfy all the abovementioned requirements. For example, the threshold voltages V(10,0,20) achieved are still too unfavorable for many applications and/or the viscosity and/or birefringence values are too high.
Since the various components of liquid-crystal mixtures can have unfavorable effects and this sometimes occurs in an unpredictable manner, the preparation of liquid-crystal mixtures which are required to satisfy various requirements simultaneously is to date a difficult task for the person skilled in the art.
For this reason, the liquid-crystal mixtures known hitherto frequently have one or more of the following disadvantages: an excessively high threshold voltage V(10,0,20), high temperature dependence of the threshold voltage, an excessively high viscosity, an unfavorably high optical anisotropy xcex94n and a large viewing-angle dependence of the contrast.
A further major problem in liquid-crystal displays of the prior art is the frequency dependence of the dielectric properties and thus of the addressing voltages. A particular problem here is the temperature dependence of the effect, which is very pronounced at low temperatures (for example at xe2x88x9220xc2x0 C.).
A reduction in the frequency dependence of the addressing voltages enables better multiplex addressing even at higher multiplex ratios. A reduction in the temperature dependence of the frequency dependence enables, in particular, the displays to be operated down to low temperatures.
In liquid-crystalline media for liquid-crystal displays having low addressing voltages in the range from 2 to 3 V (mux. 1:3, bias 1:3; mux. 1:8, bias 1:4 or mux. 1:6, bias 1:5), liquid-crystalline or mesogenic compounds having large dielectric anisotropies (xcex94∈=∈∥xe2x88x92∈xe2x8axa5, where ∈∥ is the dielectric constant parallel to the director and ∈xe2x8axa5 is the dielectric constant perpendicular to the director).
Such compounds and the media containing them typically have high viscosity values and thus long response times, i.e. switch relatively slowly.
A further problem for the low-temperature behavior is the drastically increasing viscosity of the liquid-crystal media at low temperatures, which is observed in particular in the highly polar media at low addressing voltages. The increase in viscosity, in particular in the rotational viscosity (xcex31), which is essential for switching in TN and STN displays, results in an increase in the response times which is unacceptable for many applications.
At a given temperature, (xcex94∈=∈∥xe2x88x92∈xe2x8axa5) depends on the frequency of the applied (rectangular) voltage, also referred to as the frequency of the addressing voltage or V addressing frequency for short. In a range from 100 Hz to 1000 kHz, which covers the typical addressing frequencies occurring in typical liquid-crystal displays, xcex94∈ initially remains constant with increasing frequency. At higher frequencies, xcex94∈ then decreases.
For the typical behavior, see also FIG. 3. The slight initial decrease becomes more pronounced with increasing frequency. The value of the negative slope of dxcex94∈/df increases until it reaches a maximum value at an inflection point and then decreases again. As the frequencies increase further, xcex94∈ then passes through a minimum, typically in the region of negative absolute values.
Since the deviation from constant behavior is difficult to determine experimentally and can only be measured with some error latitude (cf., for example, FIG. 3), it is better to use the frequencies which are characteristic of the frequency dependence, i.e. the frequency of the inflection point (f(xcex94∈xe2x80x2max)) (at negative slope) and of the minimum (f(xcex94∈xe2x80x2min)).
Here, owing to the better measurability and in particular owing to the position in the central region of the range in question, the frequency of the inflection point is used.
The frequency dependence of xcex94∈ causes problems in the addressing of liquid-crystal displays, since, particularly in the case of multiplex-addressed displays and in particular in the case of such displays having relatively large multiplex ratios (for example 1:16), not only are higher frequencies used, but their overtones also occur. However, if the liquid-crystal media exhibit small xcex94∈ values at the higher frequencies which occur, the contrast drops, since the characteristic voltages, for example the threshold voltage V10, depend on xcex94∈. Thus, media which are frequency-independent wherever possible up to high frequencies are desired.
With decreasing temperature, xcex94∈ increases at low frequencies. Since, however, the characteristic frequencies simultaneously drop greatly with decreasing temperature, this effect is overcompensated at high frequencies. Thus, the xcex94∈ (f,T) curves for two different temperatures T1 and T2 typically intersect at a certain frequency fx (T1, T2). The frequency of this intersection point is thus characteristic both of the frequency dependence of the medium as such and, in particular, of its temperature dependence.
The effect of the increase in xcex94∈ with decreasing temperature can be compensated relatively simply by changing the addressing voltage used.
The significantly greater frequency dependence at lower temperatures, in particular the shift in the frequency range of the decrease in xcex94∈ values, is extremely interfering and is prohibitive for many applications.
There thus continues to be a great demand for liquid-crystal mixtures having a high clearing point, low melting point, low threshold voltage V(10,0,20), small temperature dependencies of the threshold voltage and/or low frequency dependence of the dielectric anisotropy and of the threshold voltage, and low viscosity (and thus short response time) which simultaneously combine the optical anisotropies xcex94n which are suitable for use in TN and/or STN liquid-crystal cells and very small viewing-angle dependencies of the contrast with good low-temperature behaviour.
An object of the invention is to provide TN liquid-crystal displays and nematic liquid-crystal mixtures for use therein which do not have the above-mentioned disadvantages or only do so to a lesser extent, and in particular have low rotational viscosities xcex31 or short temperature dependencies of the threshold voltage d[V(10,0,T)/V(10,0,20)]/dT and/or low frequency dependence of the threshold voltage (or dielectric anisotrophy), and in particular at low temperatures.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has now been found that these objects can be achieved if liquid-crystal mixtures according to the invention are used in displays.
An object of the present invention is to provide media which have a smaller frequency dependence of xcex94∈, in particular at low temperatures.
It has now been found that liquid-crystalline media in which the inflection point of xcex94∈ as a function of the frequency f(xcex94∈xe2x80x2max, T) at T=0xc2x0 C. is 80 kHz or more are very particularly suitable for displays of this type.
It has furthermore been found that media in which the frequency dependence curves at 0xc2x0 C. and xe2x88x9220xc2x0 C. intersect at a frequency of 4 kHz or more are highly suitable for displays of this type.
It has furthermore been found that liquid-crystalline media of this type can preferably be achieved as media comprising one or more compounds of the formula I 
in which
R1 is alkyl or alkoxy having 1 to 12, preferably 1 to 7, particularly preferably 3 to 7, carbon atoms or alkenyl, alkoxyalkyl or alkenyloxy having 2 to 12, preferably 2 to 7, particularly preferably 2 to 5, carbon atoms, and
L is H or F, preferably F,
and one or more compounds of the formula II 
xe2x80x83in which
R21 is alkyl or alkoxy having 1 to 7, preferably 1 to 5, carbon atoms or alkenyl, alkoxyalkyl or alkenyloxy having 2 to 7 carbon atoms, F, Cl, OCF3 or OCF2H,
R22 is H or alkyl having 1 to 10, preferably 1 to 5, carbon atoms, particularly preferably H,
Y21 and Y22 are each, independently of one another, H or F,
n is from 0 to 5, preferably from 0 to 3, and
m is 0 or 1,
which do not have the abovementioned disadvantages or only do so to a much less pronounced degree compared with conventional media.
The terms xe2x80x9calkylxe2x80x9d and xe2x80x9calkoxyxe2x80x9d cover straight-chain and branched alkyl and alkoxy groups having 1 to 12 carbon atoms, in particular the straight-chain groups. Particularly preferred alkyl and alkoxy groups are ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl, methoxy, octoxy, nonoxy, decoxy, undecoxy or dodecoxy.
In this application, alkyl preferably denotes n-alkyl, alkenyl preferably denotes n-1-E-alkenyl, alkoxyalkyl preferably denotes alkyloxymethyl and alkoxy preferably denotes n-alkoxy.
The term xe2x80x9calkenylxe2x80x9d covers straight-chain and branched alkenyl groups having 2 to 12 carbon atoms, in particular the straight-chain groups. Particularly preferred alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl, and C7-6-alkenyl, in particular C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl. Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Of the alkenyl groups, those having 2 to 5 carbon atoms are particularly preferred.
The term xe2x80x9calkenyloxyxe2x80x9d covers straight-chain and branched alkenyloxy groups having 2 to 12 carbon atoms, in particular the straight-chain groups. It denotes in particular vinyloxy, prop-1- or -2-enyloxy, but-1-, -2- or -3-enyloxy, pent-1-, -2-, -3- or -4-enyloxy, hex-1-, -2-, -3-, -4- or -5-enyloxy or hept-1-, -2-, -3-, -4-, -5- or -6-enyloxy, furthermore oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyloxy, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyloxy, dec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyloxy, undec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8-, -9- or -10-enyloxy or dodec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8-, -9-, -10- or -11-enyloxy.
The compounds of the formulae I and II and their subformulae are prepared by methods known per se, as described in the literature (for example in the standard works, such as Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart), to be precise under reaction conditions which are known and suitable for said reactions. Use can also be made here of variants which are known per se, but are not mentioned here in greater detail.
In a first preferred embodiment, the media according to the invention are particularly suitable for improving the frequency dependence of the dielectric anisotropy and the threshold voltage. Generally preferably, and in particular in this preferred embodiment, the liquid-crystalline medium comprises the constituents described below.
The medium preferably comprises one or more compounds of the formula I in which
R1 is alkyl or alkenyl and
L is F,
in particular those in which
R1 is alkyl.
In the compounds of the formula I in the media according to the invention, R1 is preferably alkyl having 3 to 7 carbon atoms, particularly preferably having 5 to 7 carbon atoms.
In this preferred embodiment, the liquid-crystalline medium preferably comprises one or more compounds of the formula IIa 
in which
R21 and R22 are as defined under formula II, preferably
R21 is alkyl or alkenyl,
R22 is H or alkyl having 1 to 3 carbon atoms, and n is 0.
The liquid-crystalline medium particularly preferably comprises two or more compounds of the formula I and one or more compounds of the formula II and preferably of the formula IIa.
The liquid-crystalline medium especially preferably comprises one or more compounds of the formula I in which
R1 is alkenyl having 3 to 7 carbon atoms, and
L is F.
In another embodiment, the liquid-crystalline medium comprises one or more compounds of the formula I in which
R1 is alkyl having 3 to 7 carbon atoms, and
L is H or F, preferably F.
Preference is furthermore given to liquid-crystalline media which comprise one or more compounds of the formula IIa in which
R21 is alkyl, and
R22 is H.
Preference is additionally given to liquid-crystalline media which comprise one or more compounds of the formula IIa in which
R21 is alkenyl and
R22 is H.
Besides the compounds of the formulae I and II, preferably of the formula IIa, the liquid-crystalline media according to the invention comprise further liquid-crystalline or mesogenic compounds.
The liquid-crystalline media preferably comprise one or more compounds of the formula III 
in which
R31 and R32 are each, independently of one another, H, an alkyl or alkenyl radical having 1 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted by halogen, where one or more CH2 groups in these radicals may also, in each case independently of one another, be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, , xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94 in such a way that O atoms are not linked directly to one another,
A31 and A32 are each, independently of one another,
(a) a trans-1,4-cyclohexylene radical, in which, in addition, one or more non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94 and/or xe2x80x94Sxe2x80x94,
(b) a 1,4-phenylene radical, in which, in addition, one or two CH groups may be replaced by N,
(c) a radical from the group consisting of 1,4-cyclohexenylene, 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
xe2x80x83where the radicals (a) and (b) may be substituted by one or two fluorine atoms, and
Z3 is xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond, or alternatively xe2x80x94(CH2)4xe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94CH2CH2xe2x80x94,
and preferably
R31 and R32, independently of one another, are alkyl or alkoxy having 1 to 7 carbon atoms or alkenyl, alkenyloxy or alkoxyalkyl having 2 to 7 carbon atoms,
A31 and A32, independently of one another, are 1,4-trans-cyclohexylene, 1,4-phenylene, 3-fluoro-1,4-phenylene or 2,3-difluorophenylene,
Z3 is xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond.
The liquid-crystalline media particularly preferably comprise one or more compounds selected from the group consisting of the formulae IIIa-IIIc 
in which
R31 and R32, independently of one another, are alkyl or alkoxy having 1 to 7 carbon atoms or alkenyl, alkenyloxy or alkoxyalkyl having 2 to 7 carbon atoms.
The media according to the invention particularly preferably comprise one or more compounds of the formulae IIIa to IIIc in which
R31 is alkyl or alkenyl, and
R32 is alkyl, alkoxy or alkenyl.
The liquid-crystalline media according to the invention preferably comprise one or more compounds of the formula IV 
in which
R41, R42, A41, A42, A43, Z41 and Z42 each, independently of one another, are as defined above under formula III for the respectively corresponding groups R31, R32, A31, A32 and Z3.
The liquid-crystalline media according to the invention preferably comprise, as further compounds of this type, one or more compounds selected from the group consisting of the compounds of the formulae IVa and IVb 
in which
R41 and R42, independently of one another, are alkyl or alkoxy having 1 to 7 carbon atoms or alkenyl, alkenyloxy or alkoxyalkyl having 2 to 7 carbon atoms, and
L41 is H or F.
In addition, the media according to the invention preferably comprise one or more compounds of the formula V 
in which
R51, R52, A51, A52, A53, A54, Z51, Z52 and Z53 are each, independently of one another, as defined above under formula III for R31, R32, A31, A32 and Z3.
These compounds are preferably selected from the group consisting of the compounds of the formulae Va and Vb 
in which
R51 and R52, independently of one another, are alkyl or alkoxy having 1 to 7 carbon atoms or alkenyl, alkenyloxy or alkoxyalkyl having 2 to 7 carbon atoms, and
L51 is H or F.
The media according to the invention very particularly preferably comprise one or more compounds of the formula VI 
in which
R6 is alkyl having 1 to 7 carbon atoms, preferably having 2 to 5 carbon atoms, particularly preferably having 3 carbon atoms,
X is F, Cl or OCF3, preferably F, and
Y61 and Y62, independently of one another, are H or F, preferably one is F, particularly preferably both are F.
The media optionally comprise one or more compounds of the formula VII 
in which
R7 is an alkyl or alkoxy group with 1 to 12 carbon atoms or an alkenyl or alkenyloxy group having 2 to 12 carbon atoms, preferably n-alkyl having 2 to 7 carbon atoms or alkenyl having 2 to 7 carbon atoms,
X7 is CN, F, Cl, xe2x80x94CF3, xe2x80x94CF2Cl, xe2x80x94CFHCl, xe2x80x94CF2H, xe2x80x94OCF3, xe2x80x94OCF2Cl, xe2x80x94OCFHCl or xe2x80x94OCHF2, preferably CN, F, xe2x80x94OCF3 or xe2x80x94OCHF2,
l is 0 or 1, and
Y71 and Y72 are each, independently of one another, H or F, and
A71, A72, Z71 and Z72 are each, independently of one another, as defined above under formula III for A31, A32, Z31, Z32,
where the compounds of the formula VII are different from those of the formula VI.
These compounds are preferably selected from the group consisting of the compounds of the formulae VIIa to VIIc 
in which
R7 is n-alkyl having 2 to 7 carbon atoms or alkenyl having 2 to 7 carbon atoms,
X7 is F or CN, and
Y71 is H or F.
Preference is given to compounds of the formula VIIa, in particular in which X=CN.
The concentration of the compounds of the formula I in the mixture as a whole is from 10 to 50%, preferably from 20 to 40%, particularly preferably from 25 to 35%.
The concentration of the compounds of the formula II, preferably of the formula IIa, in the mixture as a whole is from 5 to 40%, preferably from 7 to 30%, particularly preferably from 9 to 25%.
The concentration of the compounds of the group of compounds of the formula III, preferably of the formulae IIIa to IIIc, is from 0 to 25%, preferably from 1 to 20%, particularly preferably from 4 to 16%.
The compounds of the formula IV, preferably of the formulae IVa to IVb, used in a total amount of from 0 to 30%, preferably from 5 to 25%, particularly preferably from 7 to 20%.
The compounds of the formula V, preferably of the formulae Va and Vb, are used in total concentrations of from 0 to 30%, preferably from 3 to 20%, particularly preferably from 5 to 15%.
If compounds of the formula VI are employed, their concentration is from 5 to 25%, preferably from 10 to 20%.
The compounds of the formula VII, preferably of the formulae VIIa to VIIc, are employed in an amount of from 0 to 20%, preferably from 5 to 15%.
Generally and in particular in a further preferred embodiment which is particularly suitable for achieving low temperature dependence, the media have the compositions and properties described below.
The nematic liquid-crystalline media which have a positive dielectric anisotropy (xcex94∈xe2x89xa71.5) preferably comprise one or more compounds of the formula Ia 
in which
R1 is as defined above under formula I,
and one or more compounds of the formula IIb 
xe2x80x83in which
R22 and n are as defined above under formula II, and
Y21 and Y22 are each, independently of one another, H or F, preferably at least one and particularly preferably precisely one of Y21 and Y22 is F.
Preferred compounds of the formula Ia are those in which R is a straight-chain alkyl radical.
Preferred compounds of the formula IIb are those in which R22 is vinyl or a straight-chain 1E- or 3E-alkenyl radical having 3 to 12, preferably having 3 to 5, carbon atoms. Of these compounds, those in which R22 is vinyl or 1E-propenyl are in turn very particularly preferred.
The use of the compounds of the formulae Ia and IIb in the mixtures for TN liquid-crystal displays according to the invention produces, in particular, low rotational viscosities xcex31 or short response times, simultaneously low threshold voltages V(10,0,20) and low temperature dependencies of the threshold voltage d[V(10,0,T)/V(10,0,20)]/dT.
Particularly when used in TN liquid-crystal displays having higher layer thicknesses, the mixtures according to the invention are distinguished by low overall response times (toverall=ton+toff)
The liquid-crystal mixtures according to the invention are furthermore distinguished by high stability and favorable values for the electrical resistance and frequency dependence of the threshold voltage. The TN liquid-crystal displays according to the invention in addition have a large working-temperature range. In particular when the TN liquid-crystal displays according to the invention are operated at the first transmission minimum, they are additionally distinguished by good angle dependence of the contrast.
Particularly when used in TN liquid-crystal displays having high layer thicknesses, the mixtures according the invention are distinguished by low overall response times (toverall=ton+toff) preferably from 40 to 70% by weight, based on the mixture as a whole.
The content of compounds of the formula Ia is preferably from 5 to 45% by weight, particularly preferably from 15 to 40% by weight, especially preferably from 20 to 40% by weight, based on the mixture as a whole.
The content of compounds of the formula IIb is preferably from 5 to 30% by weight, particularly preferably from 5 to 25% by weight, especially preferably from 10 to 25% by weight, based on the mixture as a whole.
The liquid-crystal mixtures according to the invention, in addition to the compounds of the formulae Ia and IIb, preferably comprise one or more dielectrically positive compounds (xcex94∈ greater than 1.5) selected from the group comprising the formula VI, as given above, and formula VII. 
in which the parameters are as defined above under formula VII and preferably
R7 is an alkyl or alkoxy group with 1 to 12 carbon atoms or an alkenyl or alkenyloxy group having 2 to 12 carbon atoms,
n-alkyl having 2 to 7 carbon atoms or alkenyl having 2 to 7 carbon atoms,
X7 is CN, F, xe2x80x94OCF3 or xe2x80x94OCHF2,
l is 0 or 1, and
Y71 and Y72 are each, independently of one another, H or F, and
A71, A72, Z71 and Z72 are each, independently of one another, as defined above under formula III for A31, A32, Z31 and Z32,
and preferably
Z71 is xe2x80x94COOxe2x80x94, xe2x80x94CH2CH2xe2x80x94 or a single bond, and
Z72 is xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond,
and where the compounds of formula VII are different from the compounds of the formulae I, II and VI.
Particular preference is given to compounds selected from the group consisting of the compounds of the formulae VI, VIIa to VIIc and VIId to VIIj: 
in which R7 is as defined above under the formula VII and Y71, Y72 and L5 are each, independently of one another, H or F.
Of the compounds of the formulae VI and VIla to VIIj, preference is given to the compounds of the formulae VI, VIIa and VIIf, in particular the compounds of the formula VIIa and the compounds of the formula VIIf in which one of the radicals Y71 and Y72 is F and the other of these radicals is H.
Preference is furthermore given to tricyclic compounds of the formula VII conforming to the subformulae VIIk to VIIn: 
in which R7 is as defined above under formula VII, X7 is F, Cl, CF3, OCF3 or OCHF2, Y71, Y72 and L5 are each, independently of one another, H or F, and the compounds of the formula VIIk are different from the compounds of the formula IIb and VIIb.
Particular preference is given to mixtures comprising one or more compounds of the formula VI.
Of the compounds of the formulae VIIk to VIIn, preference is given to those in which Y71 is F, furthermore those in which Y71 and Y72 are F.
Of the compounds of the formula VI, preference is given to those in which Y61 and Y62 are F.
The content of all compounds of the formulae VI, VIIa and VIId to VIIj is preferably from 5 to 35% by weight, particularly preferably from 5 to 25% by weight, based on the mixture as a whole.
The content of all compounds of formulae VIIk to VIIl is preferably from 5 to 35% by weight, particularly preferably from 5 to 25% by weight, based on the mixture as a whole.
The content of all terminally fluorinated compounds or compounds carrying terminally fluorinated groups is preferably from 5 to 65% by weight, particularly preferably from 15 to 40% by weight, based on the mixture as a whole.
The content of all compounds of the formulae VI and VII, preferably of the formulae VI and VIIa to VIIN, is preferably from 10 to 40% by weight, particularly preferably from 10 to 35% by weight, based on the mixture as a whole.
The content of all compounds of the formulae I, II, VI and VII, preferably of the formulae Ia, IIa, VI and VIIa to VIIN, is preferably from 40 to 90% by weight, particularly preferably from 50 to 80% by weight, especially preferably from 60 to 80% by weight, based on the mixture as a whole.
Besides two or more compounds selected from the formulae Ia and IIb, preferred mixtures comprise one, two, three or more compounds selected from the formulae VIIa, VIIe, VIIf and VI. These mixtures preferably comprise two to four compounds selected from the group of compounds of the formulae Ia and IIb, one or more compounds selected from the group consisting of the formulae VIIa and VIIf in which one of the radicals Y71 or Y72 is F and the other of these radicals is H, and one to four compounds of the formula VI where one of the radicals Y61 and Y62 is F and the other of these radicals is H.
In the preferred compounds, mentioned above and below, of the subformulae to the formulae III, IV, V, VI and VII, the radicals R31, R32, R41, R42, R51, R52, R6 and R7 unless stated otherwise, are preferably straight-chain alkyl, alkenyl or alkoxy, in particular alkyl having 1 to 12 carbon atoms or 1E-alkenyl having 2 to 7 carbon atoms, especially having 1 to 7 carbon atoms and 2 to 7 carbon atoms respectively.
The liquid-crystal mixtures according to the invention preferably comprise one or more compounds selected from the group consisting of the compounds of the formulae III to V.
The compounds of the formulae III to V are predominantly dielectrically neutral (xe2x88x921.5xe2x89xa6xcex94∈xe2x89xa6+1.5). They are distinguished, in particular, by their low values for the rotational viscosity xcex31.
Preferred liquid-crystal mixtures comprise one or more compounds selected from the group consisting of the compounds of the formulae III to V, preferably from 10 to 50% by weight.
The liquid-crystal mixtures according to the invention preferably comprise one or more compounds selected from the group consisting of the compounds of the formulae IIIa, IIIb and IIId to IIIj: 
in which R31 and R32 are each, independently of one another, as defined above under the formula III.
The liquid-crystal mixtures according to the invention particularly preferably additionally comprise one or more compounds selected from the group consisting of the compounds of the formulae IVa, IVb and IVc to IVo: 
in which R41 and R42 are each as defined above under the formula IV. The 1,4-phenylene groups in the compounds of the formulae IVc to IVg, IVn and IVo may each, independently of one another, also be monosubstituted or polysubstituted by fluorine.
The liquid-crystal mixtures according to the invention especially preferably additionally also comprise one or more compounds selected from the group consisting of the compounds of the formulae Va, Vb and Vc to Vg: 
in which R51 and R52 are each as defined above under the formula V, and L51 is F or H. The 1,4-phenylene groups in the compounds of the formulae Va to Vg may each, independently of one another, also be monosubstituted or polysubstituted by fluorine.
R41, R42, R51 and R52 in the compounds of the formulae IVa to IVo and Va to Vg are preferably each, independently of one another, straight-chain alkyl or alkoxy having 1 to 12 carbon atoms. Particular preference is given to compounds of the formulae IVa to IVo to Va to Vg in which R41 and R51 are alkyl and R42 and R52 are alkyl or alkoxy, each having 1 to 7 carbon atoms.
However, of the compounds of the formulae IVa to IVo, particular preference is also given to those compounds in which R41 and/or R42 are each, independently of one another, straight-chain alkenyl having 2 to 12 carbon atoms, in particular having 2 to 7 carbon atoms.
Preference is furthermore given to compounds of the formulae Va and Vg in which L is F.
In a particularly preferred embodiment, the mixtures according to the invention comprise one or more compounds selected from the group consisting of the compounds of the formulae IIIb, IVb and Vb.
In a further particularly preferred embodiment, the mixtures according to the invention comprise from about 5 to 50% by weight, preferably from 5 to 35% by weight, especially 5 to 25% by weight, of liquid-crystalline tolan compounds, for example if the liquid-crystal displays according to the invention are to be operated at the second or a higher transmission minimum. This allows smaller layer thicknesses to be used, significantly shortening the response times. The tolan compounds present in these liquid-crystal mixture: are preferably selected from the group consisting of the compounds of the formulae IIIk, VIIo, IVp and IVq: 
in which R31, R32, R41, R42 and R7 are each, independently of one another, as defined above under the formulae III, IV and VII, b is 0 or 1, X7 is F, Cl or OCF3, Z41 is xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94 or a single bond, in particular a single bond, and Lc to Lh, L5 and L6 are each, independently of one another, H or F, but where both substituents of the respective pair are not simultaneously F in any of the three substituent pairs Lc and Ld or Le and Lf or Lg and Lh.
In a further particularly preferred embodiment, the mixtures according to the invention preferably comprise from about 5 to 20% by weight of one or more dielectrically negative compounds (xcex94∈ less than xe2x88x921.5).
These compounds preferably contain the structural element 2,3-difluoro-1,4-phenylene, for example compounds as described in DE-A-38 07 801, 38 07 861, 38 07 863, 38 07 864 or 38 07 908. Particular preference is given to tolans containing this structural element as described in international patent application PCT/DE 88/00133, in particular those of the formulae IIIm and IVr 
in which R31, R32, R41 and R42 are each, independently of one another, as defined under the formulae III and IV, and Z41 is xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94 or a single bond.
Further known dielectrically negative compounds are, for example, derivatives of the 2,3-dicyanohydro-quinones or cyclohexane derivatives containing the structural unit 
as described in DE-A-32 31 707 or DE-A 34 07 013.
The individual compounds of the formulae III to VII and their subformulae and also other compounds which can be used in the displays according to the invention are either known or can be prepared analogously to the known compounds.
In further preferred embodiments, the mixtures comprise
one, two or more compounds selected from the compounds of the formula VIId in which Y71, Y72 and L5 are H, the compounds of the formula VIIe in which Y71 is H, and the compounds of the formula VIIf in which one or two of the radicals Y71 and Y72 is F and the other of these radicals is H. The radical R7 in these compounds is as defined above under formula VII and is preferably straight-chain alkyl, in particular having 2, 3, 4 or 5 carbon atoms. The content of these compounds is preferably from 0 to 60% by weight, in particular from 10 to 45% by weight, based on the mixture as a whole;
one or more, in particular one, two, three or four, compounds selected from the compounds of the formula VIIk in which X7 is OCF3 or OCHF2 and Y71 and Y72 are each, independently of one another, H or F, the compounds of the formula VIId in which Y71, Y72 and L5 are each, independently of one another, H or F, in particular in which L5 is H and Y71 and Y72 are leach, independently of one another, H or F, and the compounds of the formula VIIm in which X7 and Y71 are F and Y72 is H or F, or in which X7 is OCF3 and Y71 and Y72 are each, indepenently of one another, H or F. The radical R7 in these compounds is as defined above under formula VII and is preferably straight-chain alkyl, in particular having 2, 3, 4, or 5 carbon atoms, or straight-chain alkenyl, in particular vinyl or 1E-alkenyl having 3, 4 or 5 carbon atoms. The compounds of the formula VIIk are different from the compounds of the formula IIb. The content of these compounds is preferably from 0 to 30% by weight, in particular from 5 to 25% by weight, based on the mixture as a whole;
one or more compounds selected from the compounds of the following formulae: 
xe2x80x83in which R31, R32, R41 and R42 are each, independently of one another, straight-chain alkyl or alkoxy having 1 to 12 carbon atoms, preferably having 1 to 7 carbon atoms, or straight-chain alkenyl having 2 to 12 carbon atoms, preferably having 2 to 7 carbon atoms. The 1,4-phenylene groups in the abovementioned compounds may also be substituted by fluorine. The content of these compounds is preferably from 0 to 35% by weight, in particular from 5 to 30% by weight, based on the mixture as a whole;
one or more compounds selected from the compounds of the following formulae: 
xe2x80x83in which R51 and R52 are each, independently of one another, straight-chain alkyl or alkoxy having 1 to 12 carbon atoms, preferably having 1 to 7 carbon atoms, and L51 is H or F, preferably F. The content of these compounds is preferably from 0 to 45% by weight, in particular from 4 to 30% by weight, based on the mixture as a whole;
one or more compounds selected from the compounds of the formulae VIIo, IIIk, IVp and IVq, in particular one or more compounds selected from the compounds of the formula IIIk in which Lc to Lf are H, and the compounds of the formula IVp in which Lc to Lf are H and Z41 is a single bond. The content of these compounds is preferably from 0 to 25% by weight, in particular from 1 to 15% by weight, based on the mixture as a whole;
one or more compounds in which R1 or R21 is a trans-alkenyl group or a trans-alkenyloxy group;
one or more, in particular one, two or three, compounds of the formula VIIe in which Y71 is H, one or more, in particular one or two, compounds of the formula: VI in which Y61 and Y62 are F, one or more, in particular one or two, compounds of the formula IIIb, one or more, in particular one or two, compounds of the formula IVb, and one or more, in particular one, two or three, compounds of the formula Vb. The alkyl radicals in the compounds of the formulae VIIe and VI, IIIb, IVb and Vb are preferably straight-chain alkyl, in particular having 2, 3, 4 or 5 carbon atoms, or the radical R32 in the compounds of the formula IIIb is preferably straight-chain alkoxy, in particular having 2, 3, 4 or 5 carbon atoms.
The liquid-crystal mixtures according to the invention make it possible, while retaining the nematic phase down to xe2x88x9220xc2x0 C., preferably down to xe2x88x9230xc2x0 C., particularly preferably down to xe2x88x9240xc2x0 C., to achieve clearing points TN,Ixe2x89xa760xc2x0 C., preferably xe2x89xa765xc2x0 C., particularly preferably xe2x89xa770xc2x0 C. The nematic phase range preferably has a width of at least 85 degrees, particularly preferably at least 95 degrees. This range preferably extends from at least xe2x88x9225xc2x0 C. to +63xc2x0 C.
The shelf life in test cells at xe2x88x9220xc2x0 C., preferably at xe2x88x9230xc2x0 C., is more than 500 hours, preferably more than 1000 hours.
At the same time, the present invention makes it possible to achieve dielectric anisotropies xcex94∈ of xe2x89xa74, preferably xe2x89xa75, particularly preferably xe2x89xa78, especially preferably xe2x89xa710, very particularly preferably xe2x89xa712. The threshold voltages V(10,0,20) thus achieved are  less than 1.5 V, preferably xe2x89xa61.3 V, particularly preferably xe2x89xa61.1 V. The mixtures are thus characterized by low operating voltages Vop.
In particular in the embodiment of low frequency dependence, the threshold voltage in TN cells at dxc2x7xcex94n=0.5 xcexcm is 1.5 V or less, preferably 1.2 V or less, particularly preferably 1.05 V or less.
The average temperature dependence of the threshold voltage TN cells at dxc2x7xcex94n=0.5 xcexcm d[V(10,0,T)/V(10,0,20)]/dT of the liquid-crystal mixtures according to the invention is preferably xe2x89xa60.15%/degree, particularly preferably xe2x89xa60.08%/degree, especially preferably xe2x89xa60.05%/degree, in the range from 0xc2x0 C. to 40xc2x0 C.
The rotational viscosity xcex31 of the liquid-crystal mixtures according to the invention at 20xc2x0 C. is less than 200 mPaxc2x7s, preferably xe2x89xa6190 mPaxc2x7s, articularly preferably xe2x89xa6160 mPaxc2x7s, very particularly preferably xe2x89xa6145 mPaxc2x7s, or xe2x89xa6140 mPaxc2x7s, especially preferably xe2x89xa6120 mPaxc2x7s. The flow viscosity V20 at 20xc2x0 C. is preferably xe2x89xa660 mPaxc2x7s, particularly preferably xe2x89xa650 mPaxc2x7s.
The frequency dependence of xcex94∈ of the liquid-crystal media according to the invention is low, in particular at low temperatures. The frequency of the inflection point of xcex94∈ as a function of the frequency f(xcex94∈xe2x80x2maxT) is
at 20xc2x0 C., 250 kHz or more, preferably 300 kHz or more, particularly preferably 330 kHz or more, very particularly preferably 380 kHz or more,
at 0xc2x0 C., 80 kHz or more, preferably 160 kHz or more, particularly preferably 200 kHz or more, very particularly preferably 220 kHz or more, and
at xe2x88x9220xc2x0 C., 10 kHz or more, preferably 25 kHz or more, particularly preferably 40 kHz or more, very particularly preferably 50 kHz or more.
For the media according to the invention, the frequency at which the frequency-dependence curves intersect for given temperatures fx(T1, T2) is
at T1=0xc2x0 C. and T2=20xc2x0 C., 30 kHz or more, preferably 50 kHz or more, particularly preferably 70 kHz or more, very particularly preferably 80 kHz or more,
at T1=xe2x88x9220xc2x0 C. and T2=0xc2x0 C., 4 kHz or more, preferably 6 kHz or more, particularly preferably 8 kHz or more, very particularly preferably 10 kHz or more.
The ratio between the frequencies at the inflection points at two temperatures f(xcex94∈xe2x80x2max,T2)/f(xcex94∈xe2x80x2max,T1) of the media according to the invention is
at T1=0xc2x0 C. and T2=20xc2x0 C., 3 or less, preferably 2 or less, particularly preferably 1.7 or less, very particularly preferably 1.6 or less,
at T1=xe2x88x9220xc2x0 C. and T2=0xc2x0 C., 10 or less, preferably 8 or less, particularly preferably 6 or less, very particularly preferably 5 or less.
The liquid-crystal mixtures according to the invention are preferably suitable for operating TN liquid-crystal displays at the first Gooch and Tarry transmission minimum (C. H. Gooch, H. A. Tarry, Electron Lett. 10, 2-4, 1974; C. H. Gooch, H. A. Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975). In this case, the optical anisotropies xcex94n of the liquid-crystal mixtures according to the invention at 20xc2x0 C. are preferably in a range between 0.08 and 0.12, particularly preferably in a range between 0.09 and 0.11. However, the liquid-crystal mixtures according to the invention can also be used to operate TN liquid-crystal displays at the second or a higher transmission minimum. In this case, the optical anisotropies xcex94n of the liquid-crystal mixtures according to the invention at 20xc2x0 C. is preferably in a range between 0.10 and 0.20, particularly preferably between 0.14 and 0.16.
The person skilled in the art can set the optical anisotropy xcex94n necessary for a given layer thickness d of the TN liquid-crystal display through a suitable choice of the individual components and their proportions by weight using simple routine methods. If the value for the optical path difference dxc2x7xcex94n is specified, the value for the optical anisotropy xcex94n is determined by the layer thickness d. In particular at high values for dxc2x7xcex94n, for example if the TN liquid-crystal display according to the invention is to be operated at the second or a higher transmission minimum, the use of liquid-crystal mixtures according to the invention having a high value for the optical anisotropy xcex94n is generally preferred since the value for d can then be chosen to be relatively small, which results in better response-time values.
It goes without saying that a suitable choice of the components also allows properties of the liquid-crystal mixtures according to the invention other than the optical anisotropy xcex94n to be varied within certain limits. For example, higher clearing points TN,I can be achieved at lower dielectric anisotropies and thus higher threshold voltages V(10,0,20) or lower clearing points can be achieved at higher dielectric anisotropies and thus lower threshold voltages while retaining the other advantageous properties. Mixtures of higher xcex94∈ and thus lower thresholds can likewise be obtained even at viscosities which have been increased correspondingly little.
The liquid-crystal media according to the invention preferably comprise from 5 to 25, particularly preferably from 6 to 20, in particular from 8 to 15, compounds.
The present invention also relates to the use of the liquid-crystalline media according to the invention in electro-optical display elements for electro-optical displays.
The present invention furthermore relates to electro-optical displays, in particular TN and STN displays, which contain display elements containing a liquid-crystalline medium according to the invention.
The structure of the TN and STN displays according to the invention from polarizers, electrode baseplates and surface-treated electrodes corresponds to the conventional construction for displays of this type. The term xe2x80x9cconventional constructionxe2x80x9d here is broadly drawn and also covers all derivatives and modifications of the TN display, in particular including xe2x80x9chigh twistxe2x80x9d and xe2x80x9clow twistxe2x80x9d TN, and derivatives and modification of the STN display, such as OMI, EVA, D-STN and compensated STN (in particular film-compensated) and very particularly reflective displays.
The structure of the liquid-crystal display elements according to the invention from one or more polarizers, electrode baseplates and electrodes having such a surface treatment that the preferential direction (director) of the liquid-crystal molecules adjacent in each case are usually mutually twisted from one electrode to the other by an amount of from 70xc2x0 to 110xc2x0, corresponds to the conventional construction for display elements of this type. The term xe2x80x9cconventional constructionxe2x80x9d here is broadly drawn and also covers all derivatives and modifications of the TN liquid-crystal display.
The surface tilt angles at the two outer plates can be identical or different. Identical tilt angles are preferred. Preferred TN liquid-crystal displays have pretilt angles between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0xc2x0 to 7xc2x0, preferably from 0.01xc2x0 to 5xc2x0, in particular from 0.1xc2x0 to 2xc2x0.
The twist angle of the mixture in the TN liquid-crystal display has a value of between 22.5xc2x0 and 170xc2x0, preferably between 45xc2x0 and 130xc2x0, particularly preferably between 60xc2x0 and 115xc2x0. The twist angle especially preferably has a value of between 70xc2x0 and 110xc2x0.
An essential difference of the displays according to the invention from the conventional displays hitherto, based on the twisted nematic cell, twists, however, in the choice of the liquid-crystal parameters of the liquid-crystal layer.
The liquid-crystal mixtures which can be used in accordance with the invention are prepared in a manner conventional per se. In general, the desired amount of the components used in lesser amount are dissolved in the components making up the principal constituent, expediently at elevated temperature. It is also possible to prepare the mixtures in other conventional manners, for example by using premixtures, for example homologue mixtures, or using so-called multi-bottle systems.
The dielectrics may also contain further additives known to the person skilled in the art and described in the literature. For example, 0-15%, preferably 0-10%, of pleochroic dyes and/or chiral dopants can be added. The individual compounds added are employed in concentrations of from 0.01 to 6%, preferably from 0.1 to 3%. However, the concentration data for the remaining constituents of the liquid-crystal mixtures, i.e. the liquid-crystalline or mesogenic compounds, are given without taking into account the concentration of these additives.
The physical properties of the liquid-crystal mixtures are given in this application as determined in xe2x80x9cPhysical Properties of Liquid Crystalsxe2x80x9d, Ed. M. Becker, Merck KGaA, as of Nov. 1997, unless explicitly stated otherwise.
Above and belowxe2x80x94unless stated otherwisexe2x80x94all temperatures are given in degrees Celsius and all percentages for compositions are given in per cent by weight, the values for all physical data, including for the response times and viscosities, are based on 20xc2x0 C., and the response time is given as an average tave of the switch-on and switch-off times at the operating voltage at which the two individual values are of equal magnitude. The capacitive threshold V0 was determined at 20xc2x0 C. from dielectric constants.
The shelf life was investigated in five bonded test cells in each case (dxc2x7xcex94n=0.5 xcexcm) at fixed temperatures of 0xc2x0 C., xe2x88x9210xc2x0 C., xe2x88x9220xc2x0 C., xe2x88x9230xc2x0 C. and xe2x88x9240xc2x0 C. The cells bonded to one another on both sides with crossed polarizers were assessed visually at intervals of 24 hours. The time for stable storage at the respective temperatures tstore (T) was recorded as the final time at which no change was observed in any cell.
In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by means of acronyms, with the transformation into chemical formulae taking place in accordance with Tables A and B below. All radicals CnH2n+1 and CmH2m+1 are straight-chain alkyl radicals having n and m carbon atoms respectively. The coding in Table B is self-evident. In Table A, only the acronym for the parent structure is given. In individual cases, the acronym for the parent structure is followed, separated by a hyphen, by a code for the substituents R1, R2, L1 and L2:
Preferred mixture components are given in Tables A and B.
The mixtures according to the invention preferably comprise at least one each of two different types of compounds selected from the group of formulae given in Table B.
The mixtures according to the invention particularly preferably comprise at least one each of four different types of compounds selected from the group of formulae given in Tables A and B.
The media according to the invention preferably comprise one or more each of compounds from the group of compounds of the formulae in Table A and one or more compounds from the group of formulae in Table B.
The media according to the invention particularly preferably comprise at least four, in particular five or more, different types of compounds, which are in turn preferably selected from the group of compounds in Tables A and B.
The following abbreviations are used: